The staphylococci are by far the commonest cause of skin infections such as boils, abscesses, carbuncles and similar suppurative processes in man. They are primarily significant as pathogens. They can be grown on culture media, such as agar or meat extract media, and the individual colonies are Circular with entire edges. The pathogenic forms, that is, those isolated from suppurative processes, usually are Staphylococcus aureus.
Staphylococci ferment mannitol with the formation of acid, the latter being detected by a decrease in pH of the media, as evidenced by color change of a pH indicator such as brom thymol blue, brom cresol purple, or phenol red.
Pathogenic staphylococci, that is, the Staphylococcus aureus, also cause clotting of decalcified blood plasma due to the production of staphylocoagulase, an active clotting agent. There is a high correlation between staphylococcal virulence for man and staphylocoagulase production. Coagulase-positive bacteria are usually virulent.
Plasma clotting (i.e., staphylocoagulase activity) can be demonstrated by mixing a bacterial culture with decalcified plasma, human or rabbit, incubating the plasma and observing clotting. The simple slide method is often used for qualitative purposes; the bacteria are suspended in a drop of plasma and observed on a slide for clumping.
In the treatment of wounds and similar skin lesions it is often desirable, and sometimes necessary, to identify the microorganisms in the wound or lesion in an agar medium and the microorganisms developed therein. It is therefore desirable to be able to identify Staphylococcus aureus promptly and accurately so that appropriate treatment can be instituted without delay. For this reason it is desirable to have a culture medium, and a sensitivity paper disc which inhibits the growth of Staphylococcus aureus and allows the rapid growth of most other microorganisms and thus allows rapid and accurate identification of coagulase positive organisms, mainly Staphylococcus aureus if present in the wound or lesion.
As discussed above, two of the most widely used tests for identifying staphylococci are the coagulase production test and the mannitol fermentation test. Most strains of Staphylococcus aureus isolated from human lesions are coagulase-positive and mannitol-positive. Coagulase-negative and mannitol-negative staphylococci, i.e. S. epidermidis and S. saprophyticus are most frequently considered to be less pathogenic. Positive reactions to both the coagulase production and mannitol fermentation tests are an indication of the pathogenicity of the staphylococci. Negative reactions to both tests indicate that the isolate has less clinical significance. Classical tests for coagulase production are performed routinely by the so-called "tube" method which measures free coagulase production or by the so-called "slide" technique which measures bound coagulase either on or in the cell wall of the organism. The tube method and the slide technique are described in the publications J. Bact. 41:431-440 (1941) and Medical Microbiology, T. Cruickshank, pages 137-138, published by Williams and Wilkins Company, 11th edition, 1965, respectively. In these tests, both rabbit plasma and human plasma have been used as the substrate.
Mannitol Salt Agar is used as a selective medium for the isolation of pathogenic staphylococci. This medium is descried in J. Bact. 50:201-203 (1945). Due to the presence of 7.5% sodium chloride therein, growth of most bacteria, other than staphylococci, is inhibited on this medium. When the medium is inoculated with a material containing staphylococci and incubated for a period of 36 hours at a temperature of 37.degree. C., mannitol-fermentation staphylococci grow luxuriantly, surrounded by yellow zones. In contrast, mannitol-non fermentation staphylococci produce small colonies, surrounded by red or purple zones. Positive identification of an unknown specimen as pathogenic can require as long as five days when conventional coagulase production and mannitol fermentation tests are employed.
A solid growth medium which is suitable for the visual identification of pathogenic staphylococci from initial cultures is described in the publication Am. J. Clin. Path. 32:192-194 (August 1959). Some measure of success has been achieved using this medium. However, it has been found that other bacteria which produce coagulase, grow on this medium.